The Internet is replacing the traditional telephone network as the ubiquitous network infrastructure. With its simple network interface and global reach, the Internet is already the target of almost all new network services. While experts debate on the exact growth rate of traffic volumes on the Internet, even by the most conservative estimates, packetized IP traffic will constitute more than 95% of all network traffic within two to three years. This ever-increasing demand by consumers for greater bandwidth translates to a requirement for increased system performance. Moreover, while traditional network services continue to grow at moderate rates, all indicators predict that the much higher growth rate of Internet traffic will continue. While network operators will continue to offer service interfaces beyond IP, the inevitable and absolute dominance of IP traffic makes prudent engineering practices dictate that the network infrastructure should be optimized for IP.
However, the continuing high growth rate of Internet traffic volumes is becoming one of the Internet's most challenging scalability problems. Fiber optics using Wavelength Division Multiplexing (WDM) offers the enormous capacity that the Internet requires to continue to grow. In addition, the increasing agility of the latest Optical Layer Cross-connects (OLXCs) offers, for the first time, the ability to dynamically change the optical layer connectivity on small time scales.
OLXCs switch high bandwidth traffic. An OLXC may comprise all optical switching, or electronic switching. At the present time all optical switching does not have the ability to perform wavelength conversion. All optical cross connects may, however, change the wavelength electronically or, on the future, via all optical wavelength converters. Opto-electronic and all electronic switching perform wavelength conversion. It is important to note that OLXCs may use Time Division Multiplexing (TDM) or Wavelength Division Multiplexing (WDM). For example, an OLXC may support WDM systems with SONET framing (OC-192 bit streams) on each wavelength, switching at OC-48 granularity.
There is an underlying conflict, however, between the typical datagram (connectionless) service that supports the best-effort data delivery of the Internet and virtual circuit (connection-based) service. This conflict is exacerbated in the world of optical networks. This is due to the fixed nature of the wavelengths available and the restoration of service in optical networks.
Optical networks are connection oriented and designed for fixed rate bit streaming with very low error rates. Whereas the Internet employs soft state where possible, the state of the optical infrastructure that is encoded in its OLXCs, is hard and must be explicitly removed. The key elements in the success of the Internet have been its simplicity and the flexibility of the Internet service model. The challenge in leveraging the new optical capabilities to enhance the Internet and other services is to manage the optical resources efficiently, without sacrificing the simplicity and flexibility of the Internet.
In spite of most traffic and media types becoming IP based, high-bandwidth optical connections referred to as lightpaths will continue to be of value. Aggregate loads between major metropolitan areas are rather stable, with most of the achievable statistical multiplexing already attained in the regional and collection (distribution) part of the network. With electronic switching stretched to the limit to cope with regional network volumes, this load can conveniently be assigned to point-to-point lightpaths that bypass intermediate backbone routers, reducing their load and reducing end-to-end delay and delay variation. Traffic engineering, i.e., load and quality management, is increasingly performed by adjusting connectivity and capacity between major backbone gateways on a relatively large time-scale, still small compared to the time-scale of provisioning. This is both the primary function and the reason that ATM or Multi-Protocol Label Switching (MPLS) is being employed below the IP layer by most network operators. Agile, dynamically configurable OLXCs allow the use of the optical layer to directly implement these functions, avoiding having ATM or MPLS as intermediate layers in future networks. Lastly, lightpaths carrying transit traffic, or non-IP traffic, will remain a significant source of revenue for network operators for many years. Whereas much of the transit capacity may carry IP traffic, operators leasing optical capacity may choose not to disclose this.
There are issues involving networks in general as they relate to where particular service and intelligence are provided. For example, a connectionless network layer is used for the Internet but ATM networks have a connection-oriented network layer. Trying to run the Internet over an ATM network leads to redundancy of service at the transport layer which, for example, re-orders out-of-order packets as does the ATM layer. This is inefficient at best.
Routing algorithms are another problem. Some networks use adaptive routing algorithms and some use non-adaptive routing algorithms. Non-adaptive routing algorithms are those routing algorithms that are static and do not adjust based on the traffic, queuing delays or environmental conditions such as failed components. Connection-oriented service falls into this category for the most part. Adaptive routing algorithms are dynamic and adjust to traffic, queuing delays or environmental conditions. Datagram service in which packets do not necessarily travel the same route from source to destination are easily adjusted based on traffic, delays and environmental conditions.
This points also to issues as to where the failure or failover mechanisms lie and the response time to invoke such mechanisms. Because of the nature of datagram service, a failed component (e.g. a router) is easily overcome by re-routing. On the other hand, a similarly failed component in a connection-oriented network would lose all connections that passed through that component and even worse, it might take a significant amount of time for the source to discover the failure.
In order to prepare for the future, which will surely involve optical Internetworking, these and many other issues need to be resolved. Optical Internetworking will be a part of the future based on the amount of bandwidth that consumers are demanding.